In internal combustion engines, and in particular for spark ignition engines, reducing environmentally damaging emissions is a high priority. To this end, some engines are equipped with a secondary air system activated in a known fashion in a warming up phase to achieve rapid heating of a catalytic converter, located in an exhaust section of the internal combustion engine, to its operating temperature. The emission of the exhaust gas components HC and CO is thereby reduced in the warming up phase. Exhaust gas tests reveal a large part of the emissions of pollutants are emitted in the warming up phase of the corresponding internal combustion engine. Therefore, to reduce emissions of pollutants, the exhaust gas catalytic converter is heated as quickly as possible to its operating temperature of, for example, 500° C.
Accelerated heating of the exhaust gas catalytic converter can be achieved in a known fashion by blowing or feeding in what is referred to as secondary air into the exhaust section of the internal combustion engine. The secondary air is fed in downstream of the outlet valves of the internal combustion engine, between the start of the exhaust section and the exhaust gas catalytic converter.
A corresponding secondary air system has an electric pump for pumping air and an air valve connected downstream to control the secondary air flow. Only when the secondary air valve is at least partially open can secondary air be fed to the exhaust section. In this document, unless stated otherwise, the secondary air pump is referred to for short as “pump”, and the secondary air valve is referred to for short as “valve”.
The heating of the exhaust gas catalytic converter by means of secondary air relies on a strongly exothermic reaction of the secondary air with non-burnt fuel in the hot exhaust gas of the internal combustion engine. The further oxidation of the fuel which has not (yet) been burnt upstream and in the exhaust gas catalytic converter gives rise to accelerated heating of the exhaust gas catalytic converter to its operating temperature. In this way, the emissions of pollutants in the warming up phase of the internal combustion engine are reduced and the suitable operating temperature (referred to as “light-off” temperature) of the exhaust gas catalytic converter is reached more quickly.
DE 103 44 910 A1 discloses a diagnostic method for a secondary air system using a model pressure. The model pressure taught is a function of the following four parameters: (a) electrical supply voltage or battery voltage; (b) ambient (air) pressure; (c) ambient temperature; and (d) mass flow of the air delivered by the at (secondary) pump. This method is also carried out using a multiplicity of characteristic diagrams requiring a large amount of expenditure, in particular for setting up a secondary air system whose (secondary) pump is monitored with respect to its efficiency.